1. Field of the Invention
The present invention relates to a spoke wheel in which a rim is connected with a hub by a large number of spokes and more specifically to a spoke wheel and to spokes thereof suitably used for two-wheel vehicles such as a motorcycle and a bicycle.
2. Related Art
A wheel for use in a two-wheel vehicle is composed of a hub mounted to a wheel axis, a rim mounted with a tire and spokes that connect the hub with the rim in general. While there are two kinds of wheels of a cast wheel in which the rim, spokes and hub are unitarily molded by means of casting and a spoke wheel (or more precisely a wired spoke wheel) in which the rim is connected with the hub by a large number of wired spokes, the invention relates to the spoke wheel. The spoke wheel can absorb impacts acting on the wheel by the rim and spokes that are deformable and off-road motorcycles mostly adopt the spoke wheel.
As shown in FIGS. 8 and 9, a spoke wheel 1 is composed of a hub 3 having two flanges 2 disposed to be distant from each other in an axial direction by a predetermined distance, a rim 6 having a large number of spoke mounting concave hole portions 5 and straight wired spokes 7 each having a portion to be retained (referred to as a ‘retained portion’ hereinafter) 8 at one end thereof and a male screw portion at another end.
Each spoke 7 is tensioned between the hub 3 and the rim 6 by way of anchoring the retained portion 8 by inserting through a hole 2a of the flange 2 of the hub 3 and of fastening the male screw portion at the other end with a nipple 9 held by the concave hole portion 5 of the rim 6. The spokes 7 are tensioned at equal intervals along the whole circumference of the rim so that the four spokes in total that penetrate through the both flanges 2 of the hub 3 respectively from the outside and inside of the flanges intersect with each other and so as to have an elevation angle α as shown in FIG. 9. The spokes 7 are adjusted so that the rim 6 keeps a circular shape in plane.
While FIGS. 8 and 9 show the spoke wheel in which no tire is mounted, the spoke wheel 1 in a state in which a tire is mounted to the rim 6 may be attached to motorcycles and to off-road motorcycles for races such as motocross and Enduro in particular. While a load applied from a vehicle body acts concentratedly on a landing part S of the spoke wheel 1, a large impact load Facts on the landing part S when the racing motorcycle jumps and lands in particular. While the impact load acts so as to flatten the rim 6 through the tire and as a result, a compressive force acts on a spoke group G1 near the landing part S, tensile forces also act on spoke groups G2 and G3 neighboring the spoke group G1.
The spoke wheel 1 excels in that it can absorb the impacts described above by elastic deformation of the rim 6 and the spokes 7. When the straight spoke 7 is thinned, although stiffness of the spoke wheel drops and the shock absorption is improved, the spoke is prone to break because tensile strength of the straight spoke 7 also drops. Still more, while stiffness of the spoke wheel includes lateral stiffness against a lateral load acting during cornering, beside the vertical stiffness against the load in a direction of F (vertical direction) described above, there is a problem that the vehicle may wobble if the lateral stiffness is insufficient.
Still more, although the straight spoke 7 is fixed to the rim with the predetermined tension applied by the nipple 9 and engagement with the nipple 9 is maintained and the straight spoke 7 will not be loosened within a range in which the tension acts, the straight spoke 7 is prone to be loosened and the spoke wheel 1 may not be able to maintain an adequate shape as a spoke wheel if the compressive force acting on the spoke group G1 part exceeds the tension applied to the spokes.
Meanwhile, there is also proposed a wheel that absorbs shocks by elastic deformation of spokes by forming the spoke into an elastically deformable shape (see Japanese Patent Application Laid-open No. 2007-118936 or Utility Model Application Laid-open No. Hei.4 (1992)-37001).
When the spoke wheel 1 composed of the straight wired spokes 7 is attached to the racing off-road motorcycle in particular, longitudinal elastic modulus (Young's modulus) is relatively large because the spokes 7 are straight. Then, the spokes are prone to be loosened when large impact loads act repetitively and it is cumbersome to adjust the spokes. Still more, if the spoke is thinned to enhance the impact absorption, the spoke is prone to be broken.
The wheel disclosed in Japanese Patent Application Laid-open No. 2007-118936 is a wheel for a bicycle. The wheel has spokes each having a flexible portion formed into a shape of S for example. The wheel is arranged so as to absorb energy of vibrations by dislocating the center of a hub on which weight and others of a vehicle body acts to a lower position with respect to the center of a rim by large elastic deformation caused by the flexible portion and by alternately and repeatedly causing compressive and tensile actions on the flexible portions as the wheel rotates. This wheel cannot be applied to motorcycles and the like that receive much heavier load by any possibility.
A wheel disclosed in Utility Model Application Laid-open No. Hei.4 (1992)-37001 is a wheel for a bicycle in which a hub (mounting portion), a rim and spokes are unitarily molded by synthetic resin. The spoke is provided with an impact absorbing portion formed of an oval ringed frame to alleviate impacts because the wheel molded unitarily by synthetic resin has no shock-absorbing action. That is, this wheel is different from the spoke wheel composed of the wired spokes and having the excellent shock absorption.